Mark_LeBlanc? at discusses this.

Jean-Paul_LeBreton? also -

Fun is a general enough word to be almost meaningless. (A contrary point of view is expressed at Fun.)

Washing dishes is fun, if you're in a dishwashing competition and you have to wash more dishes than your foe. Games have to have more and better and different kinds of fun.

For example, most action-adventure videogames are trying to do much more than be fun. They have to create a new virtual experience that the player wants to have. I may be in the minority here, but I often find mini-games and sub-quests annoying. Thank God you didn't actually have to complete the fishing challenge in Zelda, for example. I don't play Zelda so I can fish. (This ties in with SimpleAsPossible.)

In other words, a game can be 'fun' and not necessarily achieve its true goals.

And it's not one of the words you should use even if "you have no words and must design."